1. Field of the Invention
The present invention relates generally to systems and methods for controlling the pilot arc current of an arc cutting or welding machine, and more particularly, to systems and methods for maintaining the pilot arc current at selected current levels or alternatively allowing the pilot arc current to track the setting for the main arc current.
2. Description of Related Art
An arc cutting or welding machine uses a torch containing an electrode and a nozzle. A path is defined between the nozzle and the electrode through which a pressurized working gas can flow and be directed toward a work piece. A current arc extends across the nozzle. The current arc is forced out of the nozzle by the pressurized gas, such that the current forms an arc at the tip of the nozzle that is akin to a flame on the end of the nozzle. The arc current has two modes. The first mode is a pilot mode, which occurs when the nozzle is away from the work piece. The second mode is a transferred or main arc current mode when the cutting system is used in cutting or welding a work piece.
FIG. 1 illustrates a configuration of a typical conventional arc cutting or welding machine. As illustrated, the torch 10 includes an electrode 10a and a nozzle 10b. A voltage source 12 is connected to the torch. The voltage source applies a negative voltage to the electrode 10a and a positive voltage to both the nozzle 10b and the work piece 14 to be cut. A gas source 16 supplies gas to the space between the electrode and nozzle.
During operation, an initial flow of gas is applied to the torch and a high frequency high voltage is applied between the electrode 10a and the nozzle 10b, whereby a spark discharge occurs. This spark discharge induces a pilot arc 18 between the electrode 10a and the nozzle 10b. The formation of the pilot arc creates a closed circuit path starting from the positive terminal of the voltage source 12 and passing through the nozzle 10b, the pilot arc, the electrode 10a, and finally returning to the negative terminal of the voltage source. When the torch is placed near the work piece 14, a part of the pilot arc 18 current begins to flow toward the work piece 14, whereby a main arc 20 is created. At this point, the pilot arc between the nozzle and electrode is replaced by the main arc between the electrode and work piece. When a cutting or other required operation is complete, a stop signal is applied to the voltage source so as to terminate the supply of power to the main arc 20, whereby the main arc 20 is extinguished. In some systems, the pilot arc is then reignited.
Most plasma torches have different sets of electrodes and nozzles for use with different types of materials to be cut or welded and/or to perform different types of cuts or welds. In general, the amount of pilot arc current in the pilot arc current mode needed to operate the torch is dependent upon the specific electrode and nozzle chosen for the cutting or welding operation. For example, one set of nozzles used to cut a thick material may be designed to operate at a higher current. In this instance, the pilot arc current would preferably have a higher current level than would be used with an electrode and nozzle configuration used at lower currents. The issue is that to date most arc cutting or welding machines do not regulate the pilot arc current. Instead, a maximum or close to maximum pilot arc current is used in all applications regardless of the type of nozzle and electrode employed. This higher pilot arc current can have deleterious effects on a nozzle and electrode designed for lower main current applications. For example, if the electrode and nozzle are designed for a lower current application, the use of a pilot arc current that is greater than what is needed can cause damage to the electrode and nozzle over time. While many manufacturers of arc cutting or welding equipment may regulate the transferred or main arc current based on nozzle type and work piece to be cut or welded, the pilot arc current is typically not regulated.
As such, systems and methods are needed that allow the pilot arc current to be altered based on either the particular nozzle being employed or the cutting operation to be performed, such that the pilot arc current is optimized for the given set-up and use of the arc cutting or welding machine.
An additional problem noted for many conventional plasma arc cutting or welding machines is deterioration of the nozzle and electrode due to rapid transition from pilot arc current to main arc current. Specifically, if the transition occurs too rapidly, the electrode may experience a condition referred to as “spitting” that deteriorates the electrode.